Computer-based imaging systems have become popular for producing both electronic and hard copy images due in part to the proliferation and availability of desktop publishing programs. In such systems, a host computer typically generates data that describes the image and then transfers the data to an image-generating device where it is converted into a format that can be used by the device. Commonly, the image data is in the format of a page description language (PDL), such as, for example, PostScript available from Adobe.
Page description language, i.e., PostScript, compatible desktop publishing and other image producing application programs generate graphic commands which are converted into page description language commands. An imaging device, such as, for example, a printer or display device interprets the page description language commands so that the proper type and sequence of graphics operations can be performed to generate or render the images created in the desktop publishing program. The graphics operations typically include text, graphics, and pictorial (bitmap) operations that are performed on image objects in response to imaging operators imbedded in the page description language. Color Render Dictionaries (CRD) are a mechanism created by Adobe to provide device independent color. Conceptually the application specifies color in a device independent way. The printer then uses its CRD to convert the device independent color to a best matching CMYK value specific to the printer.
The Color Rendering Dictionaries were designed to provide identical color for both graphics and images. In practice, the best color match for graphics is significantly different than the best color match for images. A good graphics color match will produce punchy saturated colors at the cost of matching lightness and/or hue. In contrast, a good image color match will sacrifice saturation in order to better preserve hue and lightness. Traditional approaches to generating color render dictionaries involve finding the best compromise between the color matching styles. The ability to provide a multitude of color behaviors within a single Color Render Dictionary alleviates the need to make this compromise. This results in over-all better color matching.
Beneath the interface, Adobe core code uses a CRD in two distinct ways. Graphic objects require only a single color calculation. For these sorts of objects the number of CPU cycles used to calculate the proper color is relatively small. Rendering bitmapped images may involve calculating millions of distinct colors. The number of CPU cycles used to calculate colors in images can be quite large. For this reason Adobe created two color pipelines. The accurate color pipeline is the most accurate but is relatively slow. The fast color pipeline is faster, but the resulting calculation is not as accurate. Graphic objects use the accurate color pipeline. Images use the fast color pipeline.
A critical part of switching color behaviors based on the object types consists of the task of “detecting” the object type being rendered. This question may easily be phrased as, “Am I rendering an image or a graphic object?” Postscript does not provide an explicit mechanism for making such a query. Therefore a need exists for quickly determining whether an image or graphic is desired while simultaneously rendering the appropriate color.